Trump said thanks to his intervention NATO was "much more powerful than it was two days ago", saying his demands had resulted in $US33 billion more spending on defence among the US' NATO allies than the year before, and the figure could go up to $US40 billion.

"They agreed to pay more and they agreed to pay it more quickly," he said, though he did not name countries or give a timeline.

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He did not directly answer questions about whether he had threatened to take the US out of the alliance, saying he "told people I'd be very unhappy if they didn't up their commitments very substantially... I let them know I was extremely unhappy with what was happening" and on Thursday he had been "probably a little bit more firm".

Trump said he "probably" had the power to unilaterally take the US out of NATO if he wanted to, without the approval of the US Congress.

'Do our own thing'

German news agency DPA, Politico and The Times of Londonreported the US President had openly threatened that the US would “do our own thing” unless the wealthiest allies were spending 2 per cent of their GDP on defence by January.

The Times reported he told the closed session of NATO leaders “2 per cent is a joke. 4 per cent is what people should be spending. We are being played for fools. Every single US president has been pushing the point of extra spending since Reagan and they have all been played. I am not going to be.”

Politico reported Trump warned of “grave consequences” if the allies did not immediately meet the 2 per cent spending targets agreed by the NATO members in 2014, but which only eight of the 29 allies have yet managed to achieve.

NATO officials told Politico that Trump was furious over reports that the first day of the summit in Brussels had gone smoothly, and demanded to give an immediate press conference.

NATO Secretary-General Jens Stoltenberg immediately shifted the meeting of the North Atlantic Council — the alliance’s top political body — into an allies-only emergency session.

Trump, with Secretary of State Mike Pompeo and National security adviser John Bolton, waves as he walks off stage.Credit:AP

On Wednesday US Secretary of State Mike Pompeo called NATO “the most successful alliance in history" and said all NATO allies had committed to extending its success.

But Trump has repeatedly complained about allied spending on defence since his arrival at the summit, insisting it had to be made fair.

At the press conference, Trump said the US commitment to NATO "remains very strong, primarily because of the spirit they have and the amount of money they're willing to spend, the additional money they're putting up".

He said countries had agreed to increase their defence spending to the 2 per cent figure, though some delegates had to go back home to get approval for the change.

"(The) 2 per cent (figure) was a range, a goal, now it's a commitment, it was like this amorphous number out there and now it's a real commitment," he said.

Trump acknowledged that some of NATO's poorer members might have trouble meeting the figure, but said the US could "help them out a little bit" with finance, adding that the US made "by far the best" military equipment.

Trump said he wanted "peace all over the world, that's my goal", and he wanted to achieve this by building up the military, with his dream to have the best military equipment in the world and never have to use it.